Make ScrollbarThemeAura selectively invalidate scrollbar parts.

third_party/WebKit/Source/platform/scroll/Scrollbar.cpp

 /*
  * Copyright (C) 2004, 2006, 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 121 matching lines @@
 }
 
 void Scrollbar::offsetDidChange()
 {
     ASSERT(m_scrollableArea);
 
     float position = scrollableAreaCurrentPos();
     if (position == m_currentPos)
         return;
 
+    float oldPosition = m_currentPos;
     int oldThumbPosition = theme().thumbPosition(*this);
     m_currentPos = position;
-    // TODO(jbroman): The theme should provide the parts to invalidate.
-    // At the moment, the only theme that doesn't invalidate everything is Mac,
-    // which invalidates this as needed in ScrollAnimatorMac.
-    setNeedsPaintInvalidation(NoPart);
+
+    ScrollbarPart invalidParts = theme().invalidateOnThumbPositionChange(
+        *this, oldPosition, position);
+    setNeedsPaintInvalidation(invalidParts);
+
     if (m_pressedPart == ThumbPart)
         setPressedPos(m_pressedPos + theme().thumbPosition(*this) - oldThumbPosition);
 }
 
 void Scrollbar::disconnectFromScrollableArea()
 {
     m_scrollableArea = nullptr;
 }
 
 void Scrollbar::setProportion(int visibleSize, int totalSize)
@@ skipping 408 matching lines @@
         invalidParts = AllParts;
     if (invalidParts & ~ThumbPart)
         m_trackNeedsRepaint = true;
     if (invalidParts & ThumbPart)
         m_thumbNeedsRepaint = true;
     if (m_scrollableArea)
         m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
 } // namespace blink